1. Field of the Invention
The present invention relates to methods for detection of prion protein (also denoted as PrP-Sc protein) as an indicator of transmissible spongiform encephalopathies. In particular, this invention relates to non-invasive, preclinical methods for detection of prion proteins in ruminants using third eyelid lymphoid tissue. The invention further relates to monoclonal antibodies that specifically bind a conserved epitope of prion proteins in ruminants and immunoassays using the antibodies to detect prion protein in fixed, treated tissue.
2. Description of the Art
Transmissible spongiform encephalopathies (TSEs) are a heterogeneous group of fatal neurodegenerative disorders that occur in humans, ruminant herbivores, mink, and cats. Sheep scrapie is the prototype of this group. TSEs are characterized by deposition of prion proteins (also denoted as PrP-Scrapie or PrP-Sc), the infectious form of the proteins, in the central nervous system of affected individuals. Prions have been defined as small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids. The term "prion" is a contraction of the words "protein" and "infection," and prions are comprised largely if not exclusively of PrP-Sc molecules encoded by a PrP gene. Prion diseases are often called spongiform encephalopathies because of the post mortem microscopic or histopathologic appearance of the brain of an infected animal with large vacuoles in the cortex and cerebellum. Prion proteins are insoluble, protease-resistant glycoproteins resulting from post translational modification of normal mammalian glycoproteins (PrP-Cellular or PrP-C), and deposition of the prion protein, an abnormal isoform of a native cellular sialoglycoprotein, in the central nervous system is a reliable marker of TSE infection.
The most widely studied TSEs in food-producing animals include scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle (also known as "Mad Cow" disease), and chronic wasting disease (CWD) in mule deer and elk. Other TSEs in animals included transmissible mink encephalopathy (TME) in mink and feline spongiform encephalopathy (FSE) of cats. Prion diseases of humans have also been identified. These include: Creutzfeldt-Jakob Disease (CJD); Gerstmann-Straussler-Scheinker Syndrome (GSS); Fatal Familial Insomnia (FFI), and Kuru.
The transmissible agent in these diseases remains controversial. However, as noted above, an insoluble isoform, (prion or PrP-Sc) of a mammalian sialoglycoprotein (PrP-Cellular or PrP-C) is a major component in infectious material. It appears that the scrapie isoform of the prion protein (PrP-Sc) is necessary for both the transmission and pathogenesis of the transmissible neurodegenerative diseases of animals and humans (see S. B. Prusiner, Science 252:1515-1522 (1991)). A leading hypothesis is that prion diseases result from the conversion of PrP-C to Prp-Sc by a nucleation or polymerization event.
The occurrence of novel transmissible spongiform encephalopathies in cattle in the United Kingdom and Europe and in mule deer and elk in parts of the United States has emphasized the need for reliable diagnostic tests. Further, the epizootic of a TSE in cattle and its postulated relationship to a new variant of human Creutzfeldt Jakob Disease have increased public and scientific awareness of these relatively rare disorders, and have highlighted the need for preclinical detection of TSEs. Although no cases of BSE have been detected in the United States, sensitive immunohistochemical techniques and preclinical detection methods are basic for detection, surveillance, and control of TSEs.
Prion diseases can have a long incubation period. For example, in sheep it can take 3 to 5 years from the time when an animal becomes infected until it first shows disease signs. In bovine spongiform encephalopathy (BSE) it can take two to eight years from the time when an animal becomes infected until it first shows disease signs. Infected animals and humans have neither a disease-specific immune response nor consistent biochemical, hematological and gross pathological abnormalities. The early diagnosis of transmissible spongiform encephalopathies can therefore be dependent on the appearance of clinical signs, electroencephalography, or the invasive method of taking brain biopsies. Confirmation of TSEs is accomplished by postmortem microscopic or histological examination of brain tissue of suspected cases. Postmortem histopathologic diagnosis of the ruminant TSEs is based on the appearance of neuronal vacuolation, spongiform changes, gliosis, and astrocytosis. However, these can vary in intensity and anatomic location depending on the host species, the individuals, host genetics, stage of disease, and infectious source. Thus, diagnosis by histopathology alone may be equivocal in early cases and usually not possible in autolyzed tissue.
Deposition of prion protein (PrP-Sc) in the central nervous system is a reliable marker for the TSEs. Immunohistochemical detection of PrP-Sc is therefore an important adjunct to histopathology in diagnosis, surveillance, and control of TSEs. Monoclonal antibody 263K 3F4 (U.S. Pat. No. 4,806,627) detects PrP-Sc in hamsters and humans, and has received widespread use in diagnostic assays and pathogenesis studies of human TSEs. A major disadvantage is that it fails to react with PrP from sheep and cattle (R. J. Kascsak et al., Immunological Investigations 26:259-268 (1997)). Rabbit antisera reactive with ruminant Prp-Sc has the disadvantages that it cannot be standardized for widespread use due to limitations in quantity and specificity. M. Horiuchi et al. (Journal of General Virology 76:2583-2587 (1995)) describe a panel of synthetic peptides that generated monoclonal and polyclonal antibodies reactive with the PrP-Cellular, the non-disease-related protein) in immunoblots of selected sheep and cattle tissue. They did not report effectiveness for detecting the disease-related isoform, PrP-Sc. Additionally, they did not they report effectiveness in detecting either PrP-C or PrP-Sc in formalin fixed tissues.
Post mortem diagnosis of prion diseases is made using histologic and immunohistochemical assays on brain tissue. Ante-mortem testing in humans with suspected CJD is performed by immunohistochemical and histologic examination of brain biopsies. Because brain biopsy in ruminant animals is not feasible, an alternative approach, based on W. J. Hadlow et al's observation (The Journal of Infectious Diseases 146:657-664 (1982)), has been to biopsy selected lymph nodes. Hadlow et al. demonstrated that infectivity was detectable in certain lymph nodes (retropharyngeal, tonsil, mesenteric, prescapular, bronchial-mediastinal, and spleen) and the lymphoid tissue in the intestine of scrapie-infected sheep. Hadlow's studies, carried out before the discovery of the prion protein, detected infectivity by mouse inoculation. Race et al. (American Journal of Veterinary Research 53:883-889 (1992)), Ikegami et al. (Veterinary Record 128:271-275 (1991)), and van Keulen et al. (Journal of Clinical Microbiology 34:1228-1231 (1996)) performed similar surveys by Western immunoblots or immunohistochemical assay of selected lymph nodes using polyclonal antisera. Major disadvantages of these procedures include the following: sampling of these internal tissues requires expensive invasive methods including general anesthesia with its concomitant risks and recovery period; lymphoid tissues of sheep are often infected with a bacteria, Corynebacterium pseudotuberculosis, which destroys the architecture of the node and limits its use in these assays; and tonsillar tissue traps environmental antigens, including fungal antigens, some of the which cross react with PrP-Sc, giving equivocal or false positive immunohistochemical reactions which must be resolved by technically demanding Western blot analysis.
The BSE epidemic in the United Kingdom and the European community has cost producers and consumers in direct livestock losses and indirect loss of markets for beef and beef by-products, including economically important pharmaceutical products. Sheep and beef producing countries around the world are conducting costly surveillance and quarantine programs to maintain their status as BSE-free. Most importantly, data from several scientific lines of inquiry have provided strong evidence that BSE has infected humans in Great Britain. The scope of this new disease has yet to be determined.
What are needed is a practical, inexpensive, non-invasive method for detection of PrP-Sc in live animals and sensitive immunohistochemical assays to detect PrP-Sc in animal tissues and animal by-products.